When coal is gasified and used as a fuel for electric power generation, sulfur compounds (e.g., hydrogen sulfide and carbonyl sulfide) and nitrogen compounds such as ammonia are contained in the product gas. From the viewpoint of environmental protection and corrosion prevention, these compounds are removed in wet purification equipment. The hydrogen sulfide (H2S) removed in the wet purification equipment is stripped off and discharged as an off-gas containing hydrogen oxide at a high concentration (i.e., H2S off-gas). Moreover, the recovered ammonia (NH3) is similarly stripped off and discharged as an off-gas containing ammonia (i.e., NH3 off-gas). This system is more specifically described below with reference to a flow diagram shown in FIG. 3.
Referring to the flow diagram shown in FIG. 3, hydrogen sulfide present in the product gas is removed with the aid of an amine in the H2S removal step, and the hydrogen sulfide is released again from the amine. In order to effect the combustion treatment of the resulting regeneration gas containing hydrogen sulfide, the H2S off-gas has been treated in a common combustion furnace, storage type combustion furnace or the like. As the combustion apparatus used in this combustion step, a storage type combustion furnace has conventionally been chosen and used because, when hydrogen sulfide is burned therein, the amount of SO3 formed as a by-product is small.
However, storage type combustion furnaces have problems in that they require a valve mechanism for carrying out operation while changing a plurality of flow paths in order to maintain the effectiveness of heat exchange and its operating procedure is troublesome. Moreover, they are disadvantageous from the viewpoint of reliability ensuring freedom from troubles such as failure. That is, since heat exchange is effected when a gas flows through heat reservoirs, it may happen that the temperature of one heat reservoir continues to rise while the temperature of another continues to drop. Accordingly, it has been required to carry out operation while switching a plurality of valves constantly so as to change the gas inlets and outlets for heat reservoirs properly.
On the other hand, when conventional storage type combustion furnaces (at 1,000° C.) are used for the combustion treatment of NH3 off-gas, the complete combustion treatment of NH3 cannot be achieved to cause a leak of NH3 to the downstream side. Although a high combustion temperature (about 1,500° C.) is required to decompose NH3 completely, the operating temperature of storage type combustion furnaces has been limited to about 1,000° C. owing, for example, to the endurable temperature limits of heat reservoirs comprising mullite and cordierite (high-temperature ceramic materials).
Also from the viewpoint of nitrogen oxide (NOx) reduction, it is necessary to burn NH3 off-gas at a high temperature (about 1,500° C.), because the denitrification reaction of NO (formed from a portion of NH3) with NH3 is pronounced at 1,300° C. or above. On the other hand, the NOx produced in the combustion step includes fuel NOx formed from nitrogen-containing fuels such as ammonia, and thermal NOx formed from atmospheric nitrogen in high-temperature regions (e.g., flames). Since the rate of formation of thermal NOx is enhanced in higher-temperature regions, the amount of thermal NOx produced is increased at high temperatures. However, when a large amount of an ammonia-containing gas is to be treated continuously, it is necessary to use a temperature capable of decomposing NH3 completely. That is, it has been desired to develop a technique for the treatment of an ammonia-containing gas in which NH3 is treated at a temperature capable of decomposing it completely and the amount of NOx produced can be reduced.
On the other hand, a direct-burning type combustion apparatus can treat hydrogen sulfide and ammonia at very high temperatures by burning a fuel in a burner section and feeding hydrogen sulfide and ammonia thereto. In connection with this combustion apparatus, a single-stage technique for controlling, for example, the amount of oxygen introduced and thereby burning ammonia under a reducing atmosphere, for example, at a temperature in the vicinity of 1,000 to 1,200° C. has been proposed as a technique for minimizing the amount of NOx produced by the combustion of ammonia.
However, in order to maintain a high temperature of about 1,000° C. or above under a reducing atmosphere, it is necessary to burn a large amount of additional fuel. Moreover, a large-sized combustion apparatus adapted for high-temperature conditions is required, and this is not economical from the viewpoint of operation and equipment investment. Furthermore, in order to solve the above-described problems associated with storage type combustion furnaces and thereby achieve a satisfactory reduction of NOx, it is desirable to burn and decompose ammonia at a high temperature of at least 1,300° C. or above, rather than a temperature in the vicinity of 1,000° C.
When a direct-burning type combustion apparatus is used to burn and decompose ammonia at high temperatures, NOx is produced as a result of high-temperature treatment. Consequently, a suitable measure to reduce NOx with the aid of a reducing agent (e.g., NH3, H2S or CO) or the like is required.